1. Field of the Invention
The present invention relates to a system which provides energy pulses in an excimer laser system, where the circuit for the providing the energy pulse isolates the power supply of the system so that energy reflected back from the laser and pulsing circuit does not damage the power supply.
2. Description of Related Prior Art
Pulsing circuits, or pulsers, employed for the excitation of excimer lasers have historically used thyratrons and spark gaps as switching elements, because of the extremely demanding switch parameters required for the excitation of these lasers. Electrode voltages of the order of several tens of kV with voltage rise times of typically 100 ns have to be applied to the laser electrodes in order to generate stable glow discharges and achieve efficient lasing. Under direct switching conditions this requires switched currents of 10 to 20 kA with current rise times of up to 10.sup.4 A/.mu.s.
Gas phase switches, spark gaps and large thyratrons can fulfill these demanding operating conditions. For long switch lifetimes and operation at high repetition rates, however, spark gaps are not suitable and only thyratrons can be employed. In order to achieve thyratron lifetimes of &gt;10.sup.9 shots, as required in industrial applications, the thyratron has to be augmented by magnetic pulse compression techniques to reduce the energy transfer times and with it peak current and rates of rise of current. See, D. Basting, K. Hohia, E. Albers and H. M. von Bergmann, 1984, "Thyratrons with magnetic switches, the key to reliable excimer lasers," Optoelektronik, Volume 16, 1984, pp. 128-136; and I. Smilanski, S. R. Byron and T. R. Burkes, 1982, "Electrical excitation of an XeCl laser using magnetic pulse compression," Appl. Phys. Letters, Volume 40, 1982, pp. 547-548. Thyratrons can be very reliable and are widely employed in commercial high power excimer lasers. However, they do suffer from a limited service life, which significantly adds to the operating costs of the laser, and are subject to occasional misfires, especially towards the end of their useful lifetime. Several commercial applications, namely lithography and wafer production, can not tolerate any misfires, significantly adding to the already demanding switch requirements.
Recently several all-solid-state switched circuits have been developed which can be used in place of a thyratron. See, H. M. von Bergmann and P. H. Swart, 1992, "All-solid-state pulsers for high repetition rate multi-kilowatt lasers," IEEE Proceedinqs-B, Vol. 139, 123-130; and O. Kobayashi, K. Noda, T. Shimada and M. Obara, "High power repetitive excimer lasers pumped by an all solid state magnetic exciter," Proceedings SPIE (Society of Photo-Optical Engineers), Vol. 622, High Power and Solid State Lasers, 1986, 111-117. Semiconductor switches have the potential of almost unlimited service lifetime, if operated within their safe operational regime. Furthermore, they are not subject to the problem of misfires. On the other hand, operating parameters of semiconductor switches, such as thyristors, GTOs and IGBTs are severely limited in respect of maximum allowable operating voltages and maximum rates of current rise. Typical values of operating voltages are in the range of 1.2 to 3.6 kV with the maximum current rise limited to less than a few hundred A/.mu.s. The high voltages required for the excitation of excimer lasers can only be achieved by using stacks of multiple series-connected low voltage devices or by employing a high voltage step-up transformer in the circuit.
In order to increase the peak current and the rate of rise of current to an acceptable value, multiple stages of magnetic pulse compression have to be introduced to reduce the pulse duration. Thus, the pulse output by the semiconductor switch, which typically has a switching time of several micro seconds, is modified so that it is suitable to drive the laser. More specifically, electromagnetic pulse compressors of the Melville line type are generally employed to convert the pulse of several microseconds to a pulse on the order 100 ns as is required for stable discharge formation at the laser electrodes for this task. The operation and design of these pulse compressors is described in detail by W. S. Melville, "The use of saturable reactors as discharge devices for pulse generators," Proceedings of the Institution of Electrical Engineers, Part III, Radio and Communication Engineering, Volume 98, 1951, pp. 185-207.
FIG. 1 shows a typical all-solid-state switched excimer laser excitation circuit. In the prior art the power supply 10, supplies electrical energy which is stored in the capacitor C.sub.0. The prior art excitation circuit uses a pulse transformer T.sub.1 to step up the relatively low primary voltage of typically 1 to 3 kV across C.sub.0, which can be handled by the thyristor Tr.sub.1, to the required high output voltage of 20 to 40 kV. Two or more stages of pulse compression, consisting of the transfer loops C.sub.1 -L.sub.1 -C.sub.2 and C.sub.2 -L.sub.2 -C.sub.3 are employed to reduce the pulse duration to the required 100 ns over the laser electrodes.
A further drawback limiting the use of high power and high voltage thyristors, which can be employed for the excitation of a high power excimer laser, is their relatively long recovery time. The recovery time is the time required for the switch to go into the nonconducting state, once the current through the switch has been reduced to zero. Recovery times are limited by carrier diffusion times and range from a few tens of .mu.s to several hundred .mu.s. Long recovery times can limit the maximum repetition rate at which the switch can be operated. This is especially true, if linear dc charging, e.g., by switched mode power supplies, is employed for the charging of the primary energy storage capacitor C.sub.0. It is therefore desirable to apply a negative voltage to the thyristor once the switching process has been completed to decrease the recovery time and prevent a renewed turn-on of the thyristor which can lead to latching once C.sub.0 is recharged for the next pulse. This matter, however, is complicated by the fact that power supplies in general, and switched mode power supplies in particular, do not tolerate negative voltages across their output. An isolating circuit element is therefore required to isolate the power supply from the pulser during the switching and negative voltage phase.
Some previous pulser circuits have provided for some isolation by inserting an additional switch between the power supply and the primary energy storage capacitor C.sub.0. (The switch does not necessarily have to be solid state, but it is probably preferred.) However, the insertion of such an additional switch increases the complexity of the circuit. See, A. L. Keet and M. Groeneboom, 1989, "High Voltage Solid-State Pulser for High Repetition Rate Gas Laser," EPE Conference, Aachen.
Other pulsers of the prior art use a solid state switching device for transmitting energy to the pulse compression circuit and for providing for recovery of the energy reflected by the pulse compression circuit. See, for example, International Application, WO 96/25778, Inventors Daniel L. Birx et al. The later circuit has a significant drawback since it does not provide for isolation of the power supply or for using the negative charge to commutate the triggering switch.